Virus infection causes irritation of the nasal cavity, which causes you to sneeze.

The virus and the body do not "want" to do anything as the virus is not alive and the body does not sneez voluntarily. It is just a response built during millions of years of evolution and we still have it today because it increases the chance of survival.

Of course, with simple logic one might deduce that the reason is because sneezing diminishes the viral count on the body.